1. Field of the Invention
The present invention relates to an image forming system and an option device. This system includes an image forming apparatus for forming an image through an electrophotography process, performed in such as a copier, and the option device connected to the apparatus.
2. Description of the Related Art
Conventionally, an image forming apparatus, such as a copier or a laser printer, includes a multi-stage mechanism equipped with multiple sheet feeding option devices in a multi-stage configuration. This multi-stage mechanism includes a CPU mounted on a main body of the image forming apparatus, CPUs mounted on respective option devices, and signal lines for communication between the CPUs (e.g., Japanese Patent Application Laid-Open No. 2006-326861). Specific configuration of a multi-stage mechanism of conventional sheet feeding option devices is hereinafter described with reference to the drawings. FIG. 6 is a schematic diagram illustrating the multi-stage configuration of the conventional sheet feeding option devices. In FIG. 6, the image forming apparatus 1 is mounted with a CPU 1a. Each sheet feeding option device 23 is mounted with a CPU 2. In FIG. 6, three stages of the sheet feeding option devices 23 are stacked. In this configuration, sheet feeding option devices 23-a, 23-b and 23-c are mounted with CPUs 2-a, 2-b and 2-c, respectively. Each of the CPUs (CPUs 2-a, 2-b and 2-c) of the sheet feeding option devices 23 includes a RAM that is for storing after-mentioned unique address information but is not illustrated.
Here, it is assumed that the sheet feeding option device 23 that can be stacked in a multi-stage manner is used only in a single stage or in any number of stages. Accordingly, the following configuration is required. That is, the CPU 1a of the image forming apparatus 1 and the CPUs 2-a, 2-b and 2-c of the sheet feeding option devices 23 are connected to each other by bus lines for serial communication where the CPU 1a serves as a master. Through the bus lines, the CPU 1a and the CPU 2 of each sheet feeding option device 23 communicate with each other. Thus, CMD, STS and CLK lines connected to the CPU 1a of the image forming apparatus 1 are connected to the respective CPUs 2 (CPUs 2-a, 2-b and 2-c) of the sheet feeding option devices 23 in parallel. The CMD line connected to the CPU 2 of the sheet feeding option device 23 downstream of the bus line is provided with gate circuits G-a, G-b and G-c. The output terminal of the gate circuit is in a closed state so as not to transmit a signal to a downstream side until the CPU 1a of the image forming apparatus 1 recognizes the attachment state of the corresponding sheet feeding option device 23. Subsequently, communication between the CPU 2 of the sheet feeding option device 23 upstream of the bus line and the CPU 1a of the image forming apparatus 1 is established. A unique address is assigned to the corresponding CPU 2, and stored in the RAM, not illustrated. The CPU 2 then controls an output port P0 to open the output terminal of the gate circuit G. The CPU 1a can thus communicate with the CPU 2 of the sheet feeding option device 23 downstream of the bus line. Such a configuration allows the CPU 1a to correctly recognize the configuration of the sheet feeding option device 23 even in the case where the number of attached stages of the sheet feeding option devices 23 is not fixed.
However, according to the configuration illustrated in FIG. 6, if an unidentified number of sheet feeding option devices 23 are attached, the CPU 1a of the image forming apparatus 1 is required to correctly recognize the configuration of the sheet feeding option devices 23. For such recognition, unique addresses are required to be assigned to the respective sheet feeding option devices 23 for communication with the corresponding stages of these devices. Accordingly, the sheet feeding option device 23 is required to be mounted with the CPU 2 that includes the RAM for storing the unique address information from the CPU 1a. This requirement is a factor of increasing the cost.